In studies relating to the toxic effect of misonidazole (NSC-261037) in hypoxic cells, it was found that covalent attachment of C14-misonidazole metabolites to proteins occurred after activation by rat liver microsomes or purified NADPH-cytochrome C reductase. Studies on the mechanism of action of m-AMSA (NSC-249992) were extended. The metabolites of m-AMSA (m-AQI and m-AQDI) were found to be approximately 100-fold more cytotoxic than m-AMSA. Analogues of m-AMSA with substitutions in the anilino nitrogen were non-toxic, supporting the role of N-hydroxylation in the bioactivation of m-AMSA. Studies comparing the relative DNA strand breakage capability- of m-AMSA, its analogues, and its metabolites are continuing. Also, a study of the microsomal metabolism of o-AMSA (an inactive analogue of m-AMSA) was initiated to gain information on the importance of bioactivation to the cytotoxic effects of m-AMSA.